This invention relates generally to fasteners and, more particularly, to a fastener with a locking feature.
Fasteners which are used to connect structural members together in critical applications often require a locking feature to prevent the fastener from becoming loose, or the structural members from becoming separated. A locking feature is typically required where the fastener is subject to vibration, cyclic motions, or other environmental effects. A locking feature is also used for applications in which an accurate preload is required to maintain joint integrity, eliminate gaps between joined members, and increase the life of the fastener and structural members. Loosening of the fastener is not only a problem due to possible detachment of the structural members, but on vehicles such as spacecraft or aircraft, loose parts of the fastener and associated components may cause a hazard by interfering with the operation of equipment.
One such fastener having a locking feature comprises a bolt and a nut having a groove formed in one end for receiving a sealing insert. The sealing insert is internally threaded for mating with a threaded shank of the bolt. The forming of threads on the soft material of the sealing insert and matching of the threads on the insert with internal threads formed on the metal portion of the nut typically require manufacturing techniques which are time consuming and expensive. Another type of locking nut includes a sealing insert having a smooth inner diameter formed without threads. The threads are instead formed in the insert upon the first application of the nut on the shank. The insert is not as effective during subsequent installations as during its initial use.
Moreover, if the groove for receiving the sealing insert is formed in the head or a load carrying portion of the nut, the strength of the fastener is significantly reduced. The addition of material to the fastener to increase its strength increases the weight and cost of the fastener.
In addition to the drawbacks discussed above, these fasteners all require the nut to be threaded over the entire length of the threaded shank of the bolt which requires significant installation time for bolts having large diameter or long length threads.
Rapid-assembly nuts (commonly referred to as "Zip-Nuts") reduce installation time by allowing the nut to slide axially onto the bolt until seated against a base. An example of a rapid-assembly nut is described in U.S. Pat. No. 4,378,187. These nuts are advantageous where long travel is required to seat the nut. The threaded nut can be quickly positioned on a threaded bolt by application of translational force to position the nut along the bolt. Once the nut is seated, the internal threads of the nut engage the external threads of the shank of the bolt so that the nut may be torqued to its tightened position. The design of the rapid-assembly nut, however, is not conducive to common locking features such as applying adhesives, lacquer, or special sealants to the mating threads of the nut and bolt, since the sealant leaves residue on the internal surfaces of the nut and may have adverse effects on subsequent usage and functionality of the assembly. Furthermore, these sealants impact torque tension relationships which make it difficult to determine tension in the bolt by application of a torque wrench.